It struck me, as my mind was following its own little pathways this morning, that the structure of our days, even in our "post-industrial" society, is founded in the patterns and rhythms of agriculture. (This comes about because I was actually thinking about how my system just can't tolerate regular meals any more. All that food at once knocks me out.)
Think about it. Back in our hunter/forager days, we nibbled throughout the day. I mean, we had meals, but they were often as much ritual as anything else -- a community gathering, in part to socialize, in part maybe to make sure everyone was there. (Life was kind of risky in those days.) The idea of "three squares" rises with agrarian societies: you got up in the morning and had a hearty breakfast because you had to go out and work in the fields, and then broke for lunch so you could go out and work in the fields some more, and then had dinner because you had been working in the fields all day.
We still do that.